1. Field of the Invention
This invention relates to a process for producing acicular ferromagnetic metal fine particles which comprises adding a metal compound such as a metal salt to an aqueous suspension of iron .alpha.-oxyhydroxide, depositing a metal compound onto the iron .alpha.-oxyhydroxide fine particles and reducing the resulting material.
2. Description of the Related Art
Heretofore, acicular iron oxide has been mainly used as a magnetic material in the form of fine powder for magnetic recording media, but as high-performance audio cassette tape, compact video tape, etc. have been developed, it has been required to make the density of recording higher and also to increase the performance higher. Thus, while the magnetic characteristics of such tapes each have an adequate characteristic range balanced with magnetic head, it is indispensable that the coercive force (Hc), induction (remanent induction (Br)) and relative squareness (Br/Bm) are made as high as possible, respectively. As materials which can meet these characteristics, metal fine particles of a high coercive force (Hc) and a high saturation magnetization (os), obtained by heat-reducing powder composed mainly of iron oxide or iron .alpha.-oxyhydroxide in a stream of a reducing gas such as hydrogen gas have come to be used. Moreover, in order to correspond to the requirements of making the density of recording and the performance higher, the tendency is to make the ferromagnetic metal powder of still finer particles. However, when the powder is made of finer particles, sintering of the particles between one another in the calcination process and the reduction process at the time of its production is more likely to occur, and further the outer shape-retainability of metal particles of original shape of the iron .alpha.-hydroxide particles as the raw material become inferior. In order to achieve high dispersion of fine particle powder at the time of preparing magnetic paints and thereby to increase the density and performance of the recording of tapes, dispersibility of each particle is required to be higher than conventional dispersibility. Further, even if sintering is prevented and the outer shape of original particles is retained, the most important saturation magnetization in the aspect of magnetic characteristics diminishes as the particles are made finer, and also the coercive force becomes higher than a necessary value, beyond its adequate range, and further it is difficult to erase it by magnetic head. Still further, corrosion resistance which is one of the largest drawbacks of ferromagnetic metal powder is further decreased. Namely, when the particles are made finer, various problems arise with respect to the dispersibility, saturation magnetization, coercive force, corrosion resistance, etc. of ferromagnetic metal powder.
The first problem among the above-mentioned ones, that is, prevention of sintering of particles between one another has been solved according to the process disclosed in Japanese patent No. 1268089 (Japanese patent publication No. Sho 59-47004/1984). This patent discloses a process for producing acicular metal fine particles by adding a metal salt to an aqueous suspension of iron .alpha.-oxyhydroxide, depositing a metal compound onto the resulting iron .alpha.-oxyhydroxide and then reducing the resulting material, which process comprises adding an organic acid to the above-mentioned aqueous suspension of iron .alpha.-oxyhydroxide to make the pH 4.0 or less, followed by adding a metal salt, then adding ammonia to prepare a slurry having a pH of 9.0 to 11.0, maturing this slurry at a temperature of 70.degree. C. or higher, thereafter adding an aqueous solution of silicic acid, further, adding ammonia if necessary, separating the resulting slurry by filtering means or the like, drying it to obtain dry iron .alpha.-oxyhydroxide, and reducing it. However, in order to overcome the difficultly of high dispersion of fine particles at the time of preparing magnetic paints therefrom by making the particles finer, a further dispersion of each particle is required, as described above. Namely, it has become impossible to sufficiently satisfy the above-mentioned requirement only by the countermeasure of the above patent in the aspect of dispersibility.
In order to solve the individual problems directed to dispersibility, saturation magnetization, coercive force, corrosion resistance, etc., a number of proposals have so far been made, but no process has yet been found which solves these problems at the same time and satisfies superior characteristics with a good balance. In particular, according to conventional processes, there occurs notable reduction in the dispersibility of magnetic paints as the particles are made finer.